Materials, such as polycrystalline silicon (hereinafter referred to as polysilicon), metal silicides, and metals, such as aluminum, are of growing importance in semiconductor device processing. The surface roughness of such materials is a property of particular importance.
For many applications, interfaces and surfaces formed in the process of device manufacture should be as smooth as possible. For example, the interface between a given polysilicon layer and a silicon dioxide layer grown or deposited on that polysilicon layer should be smooth enough to prevent local electrical field enhancement in the device structures under bias conditions. Local electric field enhancement causes higher current flow through the oxide and low electric breakdown strength. The polysilicon to oxide interface roughness depends upon the roughness of the original surface of the polycrystalline layer on which the oxide is grown or deposited. Photolithographic processing is also more easily performed on smooth surfaces.
In copending U.S. patent application, Ser. No. 441,371, filed Nov. 12, 1982, by A. E. Widmer, et al., there is described a process for the fabrication of polysilicon layers with very low surface roughness in a low pressure chemical vapor deposition (LPCVD) system. The polysilicon layers are deposited with amorphous or mixed amorphous-crystalline structure at a temperature not exceeding 580 Celsius (C) and subsequently crystallized at higher temperatures. The root means square (rms) surface roughness (.sigma.) of both the as-deposited and annealed polysilicon layers is preferably below 20 angstroms.
U.S. Pat. Nos. 4,352,016 and 4,352,017, issued to M. T. Duffy, et al., relate to techniques for determining the surface crystalline quality of semiconductor material. The technique described in those patents is based on the discovery that the magnitude of the reflectance of heteroepitaxial silicon films at a wavelength of about 280 nanometers (nm) can be used as a quantitative measure of the crystalline quality of heteroepitaxial silicon films, provided the surface physical features, such as surface roughness, do not interfere too strongly with the measurements. In the case of both crystalline and physical imperfections the patents provide for determining the crystalline quality of the surface material by making the reflectance measurements at two different selected wavelengths, viz., 280 and 400 nm, of the incident light. The reflectance at one of the wavelengths (280 nm) is sensitive to both the physical and the crystalline perfection of the surface while the reflectance at the other wavelength (400 nm), called the reference wavelength, is not as sensitive to the crystalline perfection but is nevertheless sensitive to the physical perfection of the surface. Effects of surface physical imperfections are diminished by expressing reflectance values at 280 nm relative to reflectance values at 400 nm. In one mode of operation of the methods described in those patents, a well-polished single crystal silicon wafer serves as a reference reflector.
There is a need in the art for determining in a rapid and nondestructive manner the surface roughness of material such as polysilicon, metal silicides and metals, such as aluminum. Moreover, there is a need for determining rapidly and nondestructively the "degree of amorphism," or, simply, "amorphism" of as-deposited amorphous silicon films prepared by the methods described in the above-identified U.S. patent application, Ser. No. 441,371. Such a determination would aid in the fabrication of complex semiconductor devices.